To keep Lygus at bay: Keep an eye on surrounding crops

Normally, a wet spring is not a harbinger of good for San Joaquin Valley cotton producers. Besides the potential for delaying planting, the unanticipated moisture can create improved conditions for Lygus.

But the signs that would normally spread fear and loathing among California growers do not appear to be manifesting themselves after unusual late spring rains delayed the planting of the Valley's 600,000 acres of Pima and Acala cotton crops.

“Those of you who have been around me very long know that I can sum up the Lygus situation very quickly: If the hills are green in May, prepare to pay; if the hills are brown, our hopes abound,” says Peter Goodell, IPM advisor with the University of California Extension Service.

Speaking at the Pima Production Summit in Visalia, Calif., in late May, Goodell said he had not observed large numbers of hosts in the foothills that surround the San Joaquin Valley that some would expect after the spring rains.

“While the situation is somewhat complicated by the rainfall patterns and temperatures that occurred in late winter and early spring, it takes very specific conditions to produce a ‘perfect storm’ for early Lygus problems,” he said.

Goodell displayed a “Lygus prevention triangle,” a take-off on the old fire prevention triangle from Smoky the Bear days.

“It requires three things,” he said. “You need available hosts, the proximity of hosts to your field and the cotton has to be susceptible. These pests won't have much effect if the cotton is already past the stage where Lygus can hurt you.”

Alternate hosts may need to be closer to cotton than might be thought, according to Goodell. A recent University of Arizona paper indicates the source effect did not extend beyond one-fourth of a mile for forage alfalfa, about one-third of a mile for weeds and about one mile for seed alfalfa.

“This suggests that Lygus from even the most potent source, like seed alfalfa, travels no further than one mile,” he noted. “The actual distance is probably dependent on their vigor and physical condition when they leave the host. Presumably, the average migration distance is a mile, not 10s of miles.”

The most critical time for cotton and Lygus control is early season, he said. “What we talk about as bad Lygus years are the ones when they can come in early during the first fruiting period or pre-bloom. In years when late planting has occurred, the opportunity for compensating early losses is reduced.”

During early season, fruit retention can suffer at relatively low population densities of Lygus (three to seven insects per 50 sweeps), especially in the first five to seven fruiting branches. “This is when you should avoid panicking and avoid the use of broad spectrum insecticides because we still have aphids and whiteflies out there and they can blow back on you.”

Goodell says cotton producers may have caught a break with the good planting weather that finally arrived in late April and early May and allowed them to get back in their fields.

“Just because we're late doesn't mean we're at any particular disadvantage,” he noted. “We had a very uniform, vigorous planting period. From my vantage point, I really think that helps with insect management because by putting together this three or four week planting period, decisions are being made simultaneously across a lot of fields.

“You don't have a guy with a wheat field over here where the bugs are coming off his, and another who's harvesting and driving insects out of his. These kinds of years tend to ideal for area wide pest management particularly if we maintain the growth pattern we have.”

When Goodell spoke, mite and thrips numbers in the San Joaquin Valley had been low. “There seem to be more aphids in other crops this year than in the past, and we're seeing some other sort of odd bugs around,” he said. “So aphids are one thing I would certainly keep my finger on the pulse on.”

Whiteflies are the “wild card” in the system this year. Growers in Arizona and Mexico were hit hard by whiteflies last year while those in California saw little, if any, of the insects. “I really don't know if they could be a problem here this year, but this is another one you really want to watch carefully.

“The good news is we have some very effective insecticides to manage those pests.”

Lygus have several key summer annual weed hosts they can develop on, but tarweed, short pod mustard and tumbleweed are some of the more significant ones, according to Goodell.

The questions growers have to ask are how long are those going to be there, how many generations can Lygus produce on them and how widespread they are and how wide of an area is it?

“This year small pod mustard is the weed of choice,” he said. “You can find that all over the place. This yellow mustard looks like black mustard but blooms in May and June rather than during the winter or early spring. (Black mustard is a winter annual.) It grows mainly in patches along the roadside.

“I'm not seeing it growing anywhere close to where we're growing cotton. It seems to be in the southern part of the valley and in the northern part. The central part of the valley around Fresno, I'm seeing much less.

Tarweed, a yellow, sticky composite flower, can be found on roadsides, along rail lines and especially on rangeland on both rims of the San Joaquin Valley. It can bloom from May through July and can utilize soil moisture for a long period of time.

“Last year, we had tarweed all along the I-5 corridor,” he said. “This year, I waited as long as I could to make a projection. I looked around, and I have just not been able to find tarweed. They're blooming so I know they're out there, but they're just not very widespread.”

Goodell said fields near creeks, rivers or flooded areas might be at greater risk, but he did not expect severe infestations of Lygus in most areas during the earliest fruiting period for cotton.

“The more substantial threat will be mid-season with Lygus moving from crops and weedy crops into cotton,” he noted. “As in most years, the crops bordering your cotton field will be the most significant source of Lygus. The movement into cotton will occur when these crops or their associated weed are no longer able to support a population, especially during July.”

Goodell says growers and farm advisors should ask themselves the following questions:

What is in close proximity to their fields?

Are Lygus present in the source field?

When will the source be unable to sustain a Lygus population?

How susceptible will the cotton be when the populations moves?

“Regular, twice weekly scouting of cotton is the best defense for not getting caught by surprise,” he said. “That and knowing what's in the fields around you.”

Growers should also consider leaving uncut alfalfa strips during June, July and August to prevent wholesale movement of Lygus from alfalfa to cotton. “Alfalfa is the best reservoir for Lygus and, when managing any reservoir, it is unwise to release all the contents at one time.”